Due to lack of time, another post in ‘shorthand’:
Outgoing SMTP messages are rejected by ISP or Gateway appliance with error “Invalid sender address”
RFC 5322 requires a from field in the header of every email. Emails that were sent from LotusScript specifying an invalid entry for the Principal field caused Domino to create invalid message headers. The mandatory From field was missing.
When using Principal fields, ensure to use valid names. This are full names or abbreviated names as listed in the Domino Directory but not email addresses. Using email addresses in the principal field causes above issue even if the address is listed as an additional entry in the User Name field.